A lithium-ion secondary battery includes a cathode and an anode that are separated by a porous separator. Lithium-ion secondary batteries are produced with use of a separator roll, which includes a cylindrical core and the above separator wound around the core. A separator roll may have a defect as a result of, for example, trapping a foreign object inside the separator during the production. This necessitates inspecting the separator for a defect. In a case where in particular, a separator has a defect caused by an electrically conductive foreign object such as metal, the foreign object may cause a short circuit inside the lithium-ion secondary battery.
Patent Literature 1 discloses a defect inspection device configured to (i) emit visible light and infrared light to a surface of a sheet being conveyed and (ii) on the basis of data on an image captured in correspondence with the amount of each of the visible light and the infrared light reflected, determine whether a defect on the surface of the sheet is caused by metal.